Micro-mechanical structure and method for manufacturing the same

ABSTRACT

Provided is a micro-mechanical structure and method for manufacturing the same, including a hydrophilic surface on at least a part of a surface of the micro-mechanical structure, so as to prevent generation of an adhesion phenomenon in the process of removing a sacrificial layer to release the micro-mechanical, wherein the sacrificial layer comes into contact with the surface of the micro-mechanical structure.

BACKGROUND ART

1. Field of the Invention

The present invention generally relates to a micro-mechanical structureand a method for manufacturing the same, and more particularly, to amicro-mechanical structure, of which at least a part is configured of ahydrophilic surface in contact with a sacrificial layer to be removed,in order to prevent the micro-mechanical structure from being stuck inthe step of removing the sacrificial layer to release themicro-mechanical structure.

2. Discussion of Related Art

Conventionally, micro-mechanical elements are produced by forming amicro-mechanical structure through a surface micro-machining process,that is, through repetitive vapor-deposition and selective etchingprocesses of a structural layer and a sacrificial layer, and thenremoving only the sacrificial layer to form an air-gap, thus releasingthe micro-mechanical structure.

The micro-mechanical structure is vulnerable to an interfacial force andthe resulting adhesion, because of a relatively wide surface area incomparison with its volume, and a relatively narrow gap from theneighboring surface. Thus, there occurs a problematic adhesionphenomenon in the process of removing the sacrificial layer to releasethe micro-mechanical structure. For this reason, preventing such anadhesion phenomenon is very important for improvement of characteristicsand yield of the element [References: Tas et al., “Stiction in surfacemicro-machining,” J. Micromech. Microeng., vol. 6, pp. 385-397, 1996;and Maboudian et al., “Critical Review: Adhesion in surfacemicro-mechanical structures,” J. Vac. Sci. Technol. B, vol, 15, no. 1,pp. 1-20, January/February. 1997]. Especially, water produced in theprocess of etching the sacrificial layer to release the micro-mechanicalstructure is known to cause the adhesion phenomenon of themicro-mechanical structure. This problem will be described below indetail, taking the most general case of using silicon oxide for thesacrificial layer and etching the sacrificial layer with hydrogenfluoride (HF) by way of an example.

In the case of etching the silicon oxide sacrificial layer through achemical etching process employing HF, the process can be subdividedinto a liquid-phase etching process and a vapor-phase etching processaccording to the state of HF. The vapor-phase etching process, which isdeveloped posterior to the liquid-phase etching process, has much largerindustrial utility because of the advantages of: 1) less occurrence ofthe adhesion phenomenon; 2) high productivity caused by omission ofde-ionized water rinsing and drying processes followed in theliquid-phase etching process; and 3) low cost due to use of small amountof a high purity of HF which is expensive and causes environmentalpollution.

In the vapor-phase etching process employing HF, a mixture of a HF gasas a reacting gas, and a water vapor or alcoholic gas serving as acatalyst for chemical reaction is generally used [References: U.S. Pat.No. 6,238,580 B1, filed on December 1999, Cole et al.; U.S. PatentPublication No. 2002/0058422 A1, filed on December 2000, Jang et al.].Methyl alcohol having an evaporation point (64.5° C.), lower than that(100° C.) of water vapor is more used than the water vapor, because theformer is effective to prevent the adhesion phenomenon.

On the other hand, the reaction of the silicon oxide sacrificial layerand the HF gas results in silicon fluoride (SiF₄) and water (H₂O), as inFormula 1. In this case, silicon fluoride having a low evaporation point(−94.9° C.), is discharged in a gas state, while, in the case of waterhaving a high evaporation point (100° C.), some x is discharged in avapor state, and the remnant 2−x is condensed and left behind in aliquid state [Reference: Helms et al., “Mechanisms of the HF/H₂O vaporphase etching of SiO₂.”].SiO₂(s)+4HF(g)→xH₂O(g)+(2−x)H₂O(L)  Formula 1.

FIGS. 1 to 3 are conceptual views for explaining a micro-mechanicalstructure where an adhesion phenomenon occurs in the conventionalprocess of removing the silicon oxide sacrificial layer through the HFvapor-phase etching process.

A sample is prepared by forming, on a substrate 11, a silicon oxidesacrificial layer 22 and micro-mechanical structures 31 a and 31 btaking a cantilever shape. A HF vapor-phase etching process is performedto the sample, so that an air-gap g is formed by removal of the siliconoxide sacrificial layer 22. At this point, water in a liquid state isformed in a shape of islands 24 at a contact angle of θc<90° on surfacesof the substrate 11 and the micro-mechanical structures 31 a and 31 b,both of which are formed of a hydrophilic material. Some of the waterislands 24 get in contact with each other, thereby building up a waterbridge 25 connecting the substrate 11 and the micro-mechanicalstructures 31 a and 31 b. In this situation, the water bridge causes acapillary force F, as expressed in Equation 1, to be exerted between thesubstrate and the micro-mechanical structure.

$\begin{matrix}{F = \frac{2A\;\gamma_{la}\cos\;\theta_{c}}{g}} & {{Equation}\mspace{20mu} 1}\end{matrix}$

wherein, g is the height of the water bridge, namely, the thickness ofair-gap, γ_(1a) is the surface tension of water in the air, and θc isthe contact angle of water on a solid surface.

In this case, the capillary force has a positive value, that is, servesas an attractive force, because θc is less than 90°. If the capillaryattraction becomes larger than the co-efficient of elasticity which isrequired to deform the micro-mechanical structure, the micro-mechanicalstructures 31 a and 31 b are bent to the substrate, thereby stickingtemporarily to it. Then, even when all the liquefied water isevaporated, no air-gap g remains between the substrate 11 and the micromechanical structures 31 a and 31 b. Accordingly, both of them arepermanently stuck by a van der Waals force acting between them.

As described above, the liquefied water, which remains on the substrateand the micro-mechanical structure, causes a problem that themicro-mechanical structure sticks to a base structure such as thesubstrate [References: Offenberg et al., “Vapor HF etching forsacrificial oxide removal in surface micromachining, ElectrochemicalSoc. Fall Meet., vol. 94, no. 2, pp. 1056-1057, October 1994; Lee etal., “Dry release for surface micromachining with HF vapor-phaseetching,” J. MEMS, vol. 6, no 3, September 1997]. To prevent theliquefied water from being generated, the temperature of the substrateshould be increased, while the pressure of reaction should be decreased.However, this remarkably reduces an etching speed of silicon oxide,which results in great reduction in productivity.

SUMMARY OF THE INVENTION

The present invention is directed to a method of solving an adhesionproblem of a micro-mechanical structure, which occurs due to waterproduced in a HF etching process for removing a sacrificial layer whilemicro-mechanical elements are manufactured.

Further, the present invention is directed to a method of manufacturinga micro-mechanical structure having a thin air-gap g from a basestructure and a small co-efficient of elasticity at a high productivity.

One aspect of the present invention is to provide a micro-mechanicalstructure released by removing a sacrificial layer comprising: a firstside portion fixed on one region of an upper surface of a basestructure; and a second side portion released by removal of thesacrificial layer, the second side portion having an opposite surface tothe upper surface, wherein the upper surface and the opposite surfaceinclude at least a hydrophilic surface to maintain a contact angle ofwater produced during removal of the sacrificial layer to be less than90°, and to prevent the produced water from building up a water bridgewhich causes adhesion of the micro-mechanical structure.

Preferably, the upper surface may be a surface of a hydrophilic layerwhich is additionally formed on the base structure, and the oppositesurface may be a surface of a hydrophilic layer which is additionallyformed on the micro-mechanical structure.

The upper surface and the opposite surface may be formed of hydrophiliclayers of materials equal to or different from each other.

The hydrophilic layer, in case that the sacrificial layer is, forexample, a silicon oxide sacrificial layer, is preferred to havecharacteristics of not being etched by an HF gas. Further, thehydrophilic layer may be formed of any one selected from the groupconsisting of aluminum (Al), titanium (Ti), chromium (Cr), iron (Fe),cobalt (Co), aluminum oxide, chromium oxide, iron oxide, and cobaltoxide.

Another aspect of the present invention provides a method formanufacturing a micro-mechanical structure comprising: providing a basestructure having an upper surface; forming a sacrificial layer on thebase structure; forming the micro-mechanical structure having a firstside portion fixed on an upper surface of the base structure, and asecond side portion to be released by removing the sacrificial layer,the second side portion having an opposite surface opposite to the uppersurface; and releasing the micro-mechanical by removing the sacrificiallayer, wherein the sacrificial layer is removed by etching in such a waythat a contact angle of water produced during removal of the sacrificiallayer is less than 90°, and that the produced water does not build up awater bridge which causes adhesion of the micro-mechanical structure.

The etching of the sacrificial layer may be preferably performed at atemperature between about 25° C. and about 45° C. by using a mixture ofan HF reacting gas, an alcoholic catalytic gas (CH₃OH, C₂H₅OH, etc.) anda carrier gas for carrying the catalytic gas (N₂, Ar, etc.).

Another aspect of the present invention provides a micro-mechanicalstructure released by removing a sacrificial layer comprising: a firstside portion fixed on an upper surface of a base structure; and a secondside portion released by removing the sacrificial layer, the second sideportion having an opposite surface opposite to the upper surface,wherein at least one of the upper surface and the opposite surfacefurther includes a hydrophilic layer formed of any one selected from thegroup consisting of aluminum (Al), titanium (Ti), chromium (Cr), iron(Fe), cobalt (Co), aluminum oxide, chromium oxide, iron oxide, andcobalt oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the more particular description of apreferred embodiment of the invention, as illustrated in theaccompanying drawing. The drawing is not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.

FIGS. 1 through 3 are cross-sectional views of a micro-mechanicalstructure for an illustration of an adhesion phenomenon which occurs ina conventional process of HF vapor-phase etching for removing a siliconoxide sacrificial layer.

FIGS. 4 through 6 are cross-sectional views of a micro-mechanicalstructure for explanation of a micro-mechanical according to anembodiment of the present invention.

FIG. 7 is a cross-sectional view of a micro-mechanical structureaccording to another embodiment of the present invention.

FIGS. 8 and 9 are a cross-sectional view and a plan view of amicro-mechanical switch according to a preferred embodiment of thepresent invention, respectively.

FIGS. 10 through 18 are cross-sectional views for illustrating anexemplary process of manufacturing a micro-mechanical switch accordingto a preferred embodiment of the present invention.

FIGS. 19 and 20 illustrate a plan picture and a graph obtained by a 3Dsurface profiling system of a micro-mechanical switch formed by using ahydrophilic layer and without using a hydrophilic layer respectively.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, a micro-mechanical structure of a preferred embodimentaccording to the present invention will be described in reference withFIGS. 4 through 6. FIGS. 4 through 6 are conceptual views for explaininga micro-mechanical structure according to an embodiment of the presentinvention.

Micro-mechanical structures 31 a and 31 b are those released by removalof a sacrificial layer, which are configured in such a way that any one31 b of them is fixed on an upper surface of a base structure 11, andthe other 31 a is released by removal of the sacrificial layer. Surfacesof the micro-mechanical structure 31 a, which is opposite to the uppersurface of the base structure 11, is configured of strong hydrophilicsurface in such a way that a contact angle of water produced duringremoval of the sacrificial layer is less than 90°, and that the producedwater does not build up a water bridge connecting the surfaces oppositeto each other.

The hydrophilic surface of the base structure 11 may be a surface of thebase structure itself, or a surface of a first hydrophilic layer 41which is additionally formed on the base structure 11. Similarly, thehydrophilic surface of the micro-mechanical structure 31 a may also besurface of the micro-mechanical structure itself, or a surface of asecond hydrophilic layer 42 which is additionally formed on themicro-mechanical structure 31 a. Meanwhile, the upper surface and theopposite surface may be configured of hydrophilic layers of materialsequal to or different from each other. Preferably, each of theadditional hydrophilic layers is formed in a shape of a thin film ofaluminum (Al), titanium (Ti), chromium (Cr), iron (Fe), cobalt (Co),aluminum oxide, chromium oxide, iron oxide, or cobalt oxide, with athickness of 5-100 nm. In this case, if the base structure 11 is madeof, for example, aluminum (Al), the base structure 11 itself has thehydrophilic surface.

As described above, FIGS. 4 through 6 illustrate, by way of an example,but not limited to, the case that both the first hydrophilic layer 41 ofthe base structure 11 and the second hydrophilic layer 42 of themicro-mechanical structures 31 a and 32 b are additionally formed.Further, FIG. 7 illustrates a configuration that only the firsthydrophilic layer 41 is additionally formed on the upper surface of thebase structure 11.

According to the present invention, water in a liquid state, which iscondensed in the process of removing the sacrificial layer 22, is formedof wide and low islands 24 on the surface of the strong hydrophiliclayer at a small contact angle. Such islands do not build up a waterbridge. Therefore, the capillary force is not exerted between the basestructure 11 and the micro-mechanical structure 31 a. Moreover, evenwhen all the liquefied water is evaporated, the micro mechanicalstructure 31 a maintain a constant gap g from the base structure 11, sothat no adhesion phenomenon is basically generated.

In other words, as described with Equation 1, the capillary force has apositive value, that is, acts as an attractive force, because θc is lessthan 90°. However, according to the present invention, the adhesionphenomenon of the micro-mechanical structure 31 a is basically preventedby suppressing the generation of the water bridge.

According to Equation 1, the attractive force cannot be generated bymaintaining the contact angle of water θc formed on the surface of thebase structure 11 and on the opposite surface of the micro-mechanicalstructure 31 a to be more than 90°. In the case of θc>90°, the capillaryforce exerted by the water bridge connecting the surface of the basestructure and the micro-mechanical structure 31 a becomes a negativevalue, that is, acts as a repulsive force. As a result, although theliquefied water is evaporated, the micro mechanical structure 31 acontinues to maintain the constant gap from the base structure 11, andthus the micro-mechanical structure can be normally released.

However, to maintain this contact angle of water, the surface of thebase structure and the opposite surface of the micro-mechanicalstructure should be formed of a hydrophobic material, not a hydrophilicmaterial. Generally, the hydrophobic material may include a polymermaterial, such as self-assembled monolayer (SAM), fluorocarbon (FC) orso forth. But, such a material is in difficulties for practicalapplication on manufacturing the micro-mechanical structures, because ofproblems of weak adhesion to other materials, low thermal instability,and decrease of hydrophobic property under the oxide atmosphere.Typically, in most cases of manufacturing the micro-mechanicalstructures, the hydrophilic material, such as silicon (Si), siliconnitride (Si₃N₄), gold (Au), or aluminum (Al), is used.

In terms of measured contact angle of the hydrophilic material which isremoved by a HF vapor-phase etching process after vapor deposition ofsilicon oxide, the contact angles of aluminum, titanium, chromium andaluminum oxide are 17°, 8°, 23° and 5° respectively, all of whichmaterials have a strong hydrophilic property.

Preferably, the sacrificial layer is formed of silicon oxide. Forexample, the sacrificial layer may employ silicon oxide (e.g., thermaloxide, thermal chemical vapor deposition (CVD) oxide, plasma enhancedchemical vapor deposition (PECVD) oxide, spin-on glass (SOG), sputteredoxide or evaporated oxide), phosphosilicate glass (PSG), orboron-phosphorous-silicate glass (BPSG).

In the case of using silicon oxide as the sacrificial layer to beremoved, the HF vapor-phase etching process is performed at atemperature between about 25 and 45° C. by using a mixture containing aHF reaction gas, an alcoholic catalytic gas (e.g., CH₃OH, C₂H₅OH, etc.)and a carrier gas for carrying the catalytic gas (e.g., N₂, Ar, etc.).In this case, the hydrophilic materials, such as Si and Au, are notchemically etched by the HF reaction gas. When the sacrificial layer isremoved by the HF vapor-phase etching process after the vapor depositionof silicon oxide, the measured contact angle of water is within therange between about 50 and 80°. Such a contact angle is possiblysatisfying requirements for adhesion of the surface of the basestructure and the opposite surface of the micro-mechanical structure.

Hereinafter, the present invention will be described in more detailabout elements to which the micro-mechanical structures are applicable,in particular, a micro-mechanical switch by way of an example. FIGS. 8and 9 are a cross-sectional view and a plan view of a micro-mechanicalswitch according to a preferred embodiment of the present invention,respectively. The micro-mechanical switch is a resistive radio frequencymicro-mechanical switch driven by an electrostatic force.

The micro-mechanical switch is comprised of: a signal line 112 formed onthe substrate 111 in such a way that input and output terminals areseparated; ground lines 113 formed on opposite sides of the signal line112; a lower electrode 114 formed of any one of the ground lines 113; abias electrode 115; a post 121 formed on the bias electrode 115; amoving plate 132 formed in a cantilever shape that one end is fixed on asubstrate 111 by the post 121 and the silicon oxide sacrificial layer(not shown) surrounding the post 121 and that a body comes into contactwith the signal lines 113 and the lower electrode 114 with a siliconoxide sacrificial layer (not shown) of a pre-determined thicknessinterposed between the lower electrode 114 and the moving plate 132; anupper electrode 133 formed on the moving plate 132 to be symmetric withrespect to the lower electrode 114; and a contact pad 134 formed on theunfixed other end of the moving plate 132 to be symmetric with respectto an open gap of the signal line 112.

Meanwhile, a first hydrophilic layer 141 of Al or Ti is formed on atleast a part of the upper surface of the lower electrode 114 which isformed of Au, for example. A second hydrophilic layer 142 of aluminumoxide (Al₂O₃) is formed on the bottom surface of the moving plate 132which is formed of silicon. The materials such as Al and Ti are allconductive metals like Au forming the lower electrode 114. The materialsuch as aluminum oxide is a good insulating material like Si. Therefore,the formation of the hydrophilic layers does not affect operation of theswitch.

The first and second hydrophilic layers 141 and 142 serve not only tomaintain the contact angle of water produced when the silicon oxidesacrificial layer is removed by the etching process to be less than 90°,but also to prevent the produced water from building up the water bridgeconnecting between the first and second hydrophilic layers 141 and 142.

Then, an operation of the resistive radio frequency micro-mechanicalswitch driven by electrostatic force will be described below. First,when a predetermined voltage is applied to the bias electrode 115, sothe electrostatic force is generated between the upper electrode 133electrically connected to the bias electrode 115 and the lower electrode114 being in a ground state. The upper electrode 133 and the movingplate 132 are then bent toward the substrate 111 by the electrostaticforce. Then, the contact pad 134 is brought into contact with andconnects the input and output terminals of the signal line 112 at thesame time, so that a signal is transferred.

Because a low drive voltage is required to expand applicability of theswitch, it is important to make a gap between the lower electrode 114and the upper electrode 133 narrow. Once the voltage applied to the biaselectrode 115 is cut off, the moving plate 132 and the upper electrode133 which have been bent are linearly restored to their original shapesby an elastic restoring force. Then, the contact pad 134 is detachedfrom the signal line 112, so that the signal is no longer transferred.

Meanwhile, the following materials are generally used to manufacture theresistive radio frequency micro-mechanical switch driven byelectrostatic force: semi-insulating GaAs for the substrate 111, Au forthe signal line 112, the ground line 113 and the contact pad 134 whichhas good electrical conductivity, undoped-silicon for the moving plate132 which has chemical stability during the HF vapor-phase etchingprocess and a good electrical insulating property, and conductive metalmaterial for the bios electrode 115, the post 121 and the upperelectrode 133.

An example of the method of manufacturing the micro-mechanical switchaccording to the preferred embodiment of the present invention will bedescribed with reference to FIGS. 10 through 18.

Referring to FIGS. 10 and 11, a silicon oxide sacrificial layer 122 a isdeposited on a GaAs substrate 111 by using a vapor deposition process,such as plasma enhanced chemical vapor deposition (PECVD), radiofrequency sputtering, and E-beam evaporation. Then, a negativephotoresist layer 123 is coated on the silicon oxide sacrificial layer122 a. A patterning process is performed to define predetermined areas(i.e., a signal line, a ground line, a bias electrode, etc.) by using aphoto lithography process. The silicon oxide sacrificial layer 123 isthen removed by using reactive ion etching (RIE), so that the substrate111 is exposed.

Referring to FIG. 12, after deposition of Au electrode layers 112, 113,114 and 115 by using E-beam evaporation, the photoresist layer 123 andthe Au electrode layers formed on the photoresist layer aresimultaneously removed by using a lift-off process. This etching resultsin formation of a trench pattern of silicon oxide (SiO₂), within whichthe Au electrode layers 112, 113, 114 and 115 are formed. In this case,a titanium layer (not shown) may be formed between the Au electrodelayers 112, 113, 114 and 115 and the GaAs substrate 111 to enhanceadhesion.

Referring to FIG. 13, after defining a part of the region of the groundline 113 (i.e., the region of the lower electrode 114) through thecoating and patterning processes of the photoresist layer 123, the firsthydrophilic layer 141 having the hydrophilic surface is formed by usingTi (or Al, Cr, etc.) evaporation or lift-off process.

Referring to FIG. 14, an additional silicon oxide layer is formed on thepreviously formed silicon oxide 122 a to form a silicon oxide layer 122,and then a post 121 is formed through coating and patterning of thephotoresist, selective etching of the silicon oxide layer, vapordeposition of the Au layer, and a lift-off process.

Referring to FIG. 15, after defining a region where the signal line 112and the contact pad 134 overlaps each other through coding andpatterning of the photoresist, a groove H is formed by etching thesilicon oxide sacrificial layer having a pre-determined thickness.

Referring to FIG. 16, after defining a predetermined region through thecoating and patterning of the photoresist, the moving plate 132 and thesecond hydrophilic layer 142 formed of Al₂O₃ having a strong hydrophilicproperty are formed by using the E-beam evaporation and the lift-offprocess.

Referring to FIG. 17, after defining a predetermined region through thecoating and patterning of the photoresist, the Au upper electrode 133and the contact pad 134 are formed by using the E-beam evaporation andthe lift-off process.

Referring to FIG. 18, the silicon oxide sacrificial layer 122 is removedby using the HF vapor-phase etching process. The HF vapor-phase etchingprocess is performed by using a mixture of the HF reaction gas, thealcoholic catalytic gas (CH₃OH, C₂H₅OH, etc.) and carrier gas forcarrying the catalytic gas (N₂, Ar, etc.) et the temperature betweenabout 25 and 45° C. Herein, an etching hole, which is not shown in FIGS.10 through 18, may be formed in the moving plate 132 and the Au upperelectrode 133 to facilitate removing of the silicon oxide sacrificiallayer 122.

COMPARATIVE EXAMPLE

An experiment was conducted with the foregoing micro-mechanical switchmanufactured on a real scale to observe the adhesion phenomenon of themicro-mechanical structures. Each of FIGS. 19 and 20 illustrates a planpicture and a graph obtained by a 3D surface profiling system of themicro-mechanical switch formed by using a hydrophilic layer and withoutusing a hydrophilic layer respectively. The micro-mechanical switchhaving the hydrophilic layer had the same structure as that describedwith reference to FIGS. 8 and 9.

The detailed manufacturing conditions will be described with referenceto FIG. 8. The substrate 111 was made of insulating gallium-arsenic(GaAs), the signal line 112 and the lower electrode 114 were of an Authin film having the thickness of 0.5 μm, the moving plate 132 was of aSi thin film having the thickness of 0.3 μm, and the upper electrode 133was of an Au thin film having the thickness of 0.9 μm. The thickness ofthe silicon oxide sacrificial layer 122 between the lower electrode 114and the moving plate 132 was configured to be very thin, that is, 0.5μm, so that the switch may have a low drive voltage. The firsthydrophilic layer 141, which was made of aluminum of 0.05 μm thickness,was formed on the upper surface of the lower electrode 114 made of an Authin film of 0.45 μm thickness. The second hydrophilic layer 142, whichwas made of aluminum oxide of 0.05 μm thickness, was formed on thebottom surface of the moving plate 132 made of a Si thin film of 0.25 μmthickness.

The graph in the lower part of FIG. 19 is the result of observing theswitch by 3D surface profiling system, after removing a silicon oxidesacrificial layer through a HF vapor-phase etching process. As shown inthe graph, the height from the surface of the lower electrode 114 to thesurface of the upper electrode 133 was 2.0˜5.1 μm, which is larger thanthe thickness 1.2 μm of the micro-mechanical structure in the shape of acantilever comprising three layers of the second hydrophilic layer 142,the moving plate 132 and the upper electrode 133. That means that themicro-mechanical structure is being desirably released without anyoccurrence of adhesion. For reference, the appearance of the bent areain the graph is due to residual stress, which is not relevant with thepresent invention.

The micro-mechanical switch having no hydrophilic layer was manufacturedin the same configuration with the above switch having a hydrophiliclayer except the first and the second hydrophilic layer. Referring toFIG. 20, the height between the surfaces of the substrate 111 and thelower electrode 114 and the surface of the upper electrode 133 was 1.2μm, which is equal to the sum of the thickness 0.3 μm of the movingplate 132 and the thickness 0.9 μm of the upper electrode 133. Thatmeans that the micro-mechanical structure, which is formed in the shapeof a cantilever comprising two layers of the moving plate 132 and theupper electrode 133 formed on the moving plate.

The inventors of the present invention have observed the contact angleof water on the surface, after removing the sacrificial layer of by anetching process to find out the reason of an adhesion phenomenon of amicro-mechanical switch, while using the several materials for thesacrificial layer. As a result, it has been found that GaAs had 19° asthe contact angle of water, Si and Au had 57° and 62° respectively. Inother words, while GaAs has a strong hydrophilic property, Si and Auhave a weak hydrophilic property. Therefore, when a silicon oxidesacrificial layer is removed by a HF vapor-phase etching process, awater bridge having the contact angle of less than 90° is likely to beproduced between a GaAs substrate and a Si moving plate, between an Aulower electrode 114 and a Si moving plate 132, and between an Au signalline 112 and an Au contact pad 134. Especially, such a water bridge isvery likely to be produced between a Si moving plate 132 and an Au lowerelectrode 114, the gap of which is relatively small and the facing areaof which is relatively large. Therefore, the adhesion phenomenon of theswitch occurs by the capillary force generated by the water bridgehaving the contact angle of less than 90°.

Meanwhile, the contact angles of Al, Ti and aluminum oxide were lessthan 90°, about 8°, about 5°, respectively. They had a stronghydrophilic property, and chemical stability not to be removed by the HFvapor-phase etching process. Therefore, in the micro-mechanical switchshown in FIG. 19, when a silicon oxide sacrificial layer is removed bythe HF vapor-phase etching process, the water bridge in the liquid stateis not produced because of the strong hydrophilic surface of thestructure surrounding the silicon oxide sacrificial layer, and therebythe adhesion phenomenon does not occur.

As described above, according to the present invention, in amicro-mechanical structure released by removing a sacrificial layer, theadhesion phenomenon of a micro-mechanical structure in a removal processof the sacrificial layer can be prevented. Thereby, it is possible todecrease the gap between a base structure and a micro-mechanicalstructure, and thus the high yield of a micro-mechanical structure ofsuperior characteristic is obtained.

Preferred embodiments of the present invention have been disclosedherein and, although specific terms are employed, they are used and areto be interpreted in a generic and descriptive sense only and not forpurpose of limitation. Accordingly, it will be understood by those ofordinary skill in the art that various changes in form and details maybe made without departing from the spirit and scope of the presentinvention as set forth in the following claims.

1. A micro-mechanical structure released by removing a sacrificiallayer, comprising: a first side portion fixed on one region of an uppersurface of a base structure; and a second side portion released byremoval of the sacrificial layer, the second side portion having anopposite surface to the upper surface, wherein the upper surface and theopposite surface include at least a hydrophilic surface to maintain acontact angle of water produced during removal of the sacrificial layerto be less than 90°, and to prevent the produced water from building upa water bridge which causes adhesion of the micro-mechanical structure.2. The micro-mechanical structure according to claim 1, wherein theupper surface is a surface of a hydrophilic layer which is additionallyformed on the base structure.
 3. The micro-mechanical structureaccording to claim 1, wherein the opposite surface is a surface of ahydrophilic layer which is additionally formed on the micro-mechanicalstructure.
 4. The micro-mechanical structure according to claim 2 or 3,wherein the hydrophilic layer has a thickness of 5˜100 nm.
 5. Themicro-mechanical structure according to claim 1, wherein the uppersurface and the opposite surface are formed of hydrophilic layers ofmaterials equal to or different from each other.
 6. The micro-mechanicalstructure according to claim 1, wherein the hydrophilic surface isformed of any one selected from the group consisting of aluminum (Al),titanium (Ti), chromium (Cr), iron (Fe), cobalt (Ca), aluminum oxide,chromium oxide, iron oxide, and cobalt oxide.
 7. The micro-mechanicalstructure according to claim 1, wherein the sacrificial layer is formedof silicon oxide.
 8. A method for manufacturing a micro-mechanicalstructure, comprising: providing a base structure having an uppersurface; forming a sacrificial layer on the base structure; forming themicro-mechanical structure having a first side portion fixed on an uppersurface of the base structure, and a second side portion to be releasedby removing the sacrificial layer, the second side portion having anopposite surface opposite to the upper surface; and releasing themicro-mechanical by removing the sacrificial layer, wherein thesacrificial layer is removed by etching in such a way that a contactangle of water produced during removal of the sacrificial layer is lessthan 90° on surfaces of the base structure and the micro-mechanicalstructure, and that the produced water does not build up a water bridgewhich causes adhesion of the micro-mechanical structure.
 9. The methodaccording to claim 8, wherein the sacrificial layer is formed of any oneselected from a silicon oxide layer, a phosphosilicate glass (PSG)layer, and a boron-phosphorous-silicate glass (BPSG) layer, the siliconoxide layer including any one of thermal oxide, thermal chemical vapordeposition (CVD) oxide, plasma enhanced chemical vapor deposition(PECVD) oxide, spin-on glass (SOG), sputtered oxide and evaporatedoxide.
 10. The method according to claim 9, wherein the sacrificiallayer is etched at a temperature of about 25° and about 45° by a mixtureof an HF reaction gas, an alcoholic catalytic gas and a carrier gas forcarrying the catalytic gas.
 11. The method according to claim 8, whereinthe upper surface and the opposite surface are hydrophilic surfacesformed of materials equal to or different from each other, respectively.12. The method according to claim 11, wherein the hydrophilic surface isformed of any one selected from the group consisting of aluminum (Al),titanium (Ti), chromium (Cr), iron (Fe), cobalt (Co), aluminum oxide,chromium oxide, iron oxide, and cobalt oxide.
 13. The method accordingto claim 8, wherein the upper surface is a surface of a hydrophiliclayer which is additionally formed on the base structure.
 14. The methodaccording to claim 10, wherein the opposite surface is a surface of ahydrophilic layer which is additionally formed on the micro-mechanicalstructure.
 15. A micro-mechanical structure released by removing asacrificial layer, comprising: a first side portion fixed on an uppersurface of a base structure; and a second side portion released byremoving the sacrificial layer, the second side portion having anopposite surface opposite to the upper surface, wherein at least one ofthe upper surface and the opposite surface further includes ahydrophilic layer formed of any one selected from the group consistingof aluminum (Al), titanium (Ti), chromium (Cr), iron (Fe), cobalt (Co).16. The micro-mechanical structure according to claim 15, wherein thehydrophilic layer has a thickness of 5˜100 nm.